Please help me. We are having trouble with the zen cart payment module.

The situation is this:

1 In full payment mode, everything works fine until the buyer is returned to the cart after the completion of the sale.
2 At that point, they receive a message saying their credit card address didn't match the address they entered when they joined.
3 All aspects of the transaction function OK, even though there's that error message.
4 This lead the ANZ eGate guy to say it must be because in the USA they match card addresses, but in Australia they don't. So when it ch3ecks to match the addresses, it generates the secure hash error.

How can I fix this problem? Anyone knows?